Electronic door lock and supervisory system



April 29, 1969 c. v. CRANE 3,441,808

ELECTRONIC DOOR LOCK AND SUPERVISORY SYSTEM Filed Oct. 2:, 1965 Sheet Iof 2 r 5 I M m g 5% Q g I Q I Q 5 rx x 8 Q n S g? 4 v, l

wwwwi i a *11 Q1 1 '1 1 1 1 1 1 1 1 u Char/e5 1 Cmxre BYZkZ &0 ATTORNEYSELECTRONIC DOOR LOCK AND SUPERVISORY SYSTEM Filed Oct. 2:5, 1965 C. V.CRANE A lrn 29, 1969 Sheet gw --H mvsm'on Chan/e5 M d/vwe United StatesPatent 3,441,sos ELECTRONIC DOOR LOCK AND SUPERVISORY SYSTEM Charles V.Crane, 605 W. Oak, Olathe, Kans. 66061 Filed Oct. 23, 1965, Ser. No.503,618 Int. Cl. Hillh 47/14 U.S. Cl. 317-134 16 Claims ABSTRACT OF THEDISCLOSURE An electronic control and supervisory system provides controland supervision of a normally locked access door. The door can only beopened by those having knowledge of a predetermined arbitrarycombination" or code. The system has a plurality of bistable flip-flopcircuits which are either in a register or a reset condition, dependingupon the selection of certain digit switches, however, prior toutilization of the switches, all circuits are simultaneously placed inthe reset condition. The flip-flops, when all are in the registercondition, enable a gate circuit which is used to assist in theactivating of a lock releasing door relay circuit.

In business, industry, research and other fields of activity there aremany plants and institutions which have rooms or sections of theirpremises that are reserved exclusively for the use of authorizedpersonnel, it being important that all others be kept out. Frequently,those authorized to have access to a given department or area of thepremises are among those denied access to one or more other restrictedareas, and vice versa.

It is an object of the invention to provide an improved system forenabling authorized personnel, without the use of a key, easily to opena door giving access to the area they are authorized to enter, whilepreventing the door from being opened by anyone not familiar with anarbitrary code or combination known to said authorized personnel.

According to the invention, the code which must be known in order toopen the door from the outside is a multi-digit code or the equivalent,and can have as many digits as desired. An object of the invention is toprovide improved means for requiring the person seeking entry not onlyto register the correct digits of the code but also to register themseriatim in the correct order.

Another object is to require the person seeking entry to complete thecorrect code within a predetermined interval of time after coderegistration is initiated by him, this time interval being sufiicientlyshort to prevent completion of the required code except by thosefamiliar with it in advance.

A further object of the invention is to provide other effectivesafeguards against the possibility of unauthorized personnel gainingentry, or gaining knowledge of the code, through studied or randommanipulation of the code registration facilities. In this connection, asalient feature resides in providing for automatic but undisclosederasure of correct digital registration if accompanied by an incorrectdigit or by an error in the digital sequence.

Another object is to provide means whereby a security ofiicer or othersupervisory personnel can very quickly and easily change the code orcombination which must be known and used by those desiring to enter arestricted area via the combination-controlled door. A related object'ice is to provide a control circuit having a jack adapted to receiveinterchangeable plugs, the different plugs when inserted serving toestablish different codes which will serve to unlock the door.

Still another object is to provide a door-control system of thecharacter indicated having monitoring facilities whereby a securityofficer at a remote location is instantaneously apprised of everyopening and every closing of the door so that he is fully informed as toits condition at all times; and whereby he also is automaticallyinformed of every unsuccessful attempt to open the door so that suchattempts can be investigated without delay, as and when they occur.

Other objects and features of the invention will appear in the course ofthe following description thereof.

For purposes of description, it will be convenient to refer to theaccompanying drawings, wherein FIGURES l and 2, together, comprise aschematic circuit diagram of a control and supervisory system embodyingthe invention; these two figures should be positioned edge-toedge withFIGURE 2 adjacent to the lower edge of FIG- URE 1.

Referring more particularly to the drawings, M is a monitoring stationwhich may be assumed to be situated at a location remote from the doorwhose operation is to be supervised. In a convenient place outside thedoor in question, for example on the wall adjacent thereto, there is asmall panel or other suitable box A having a group of switches andlamps, the use of which will be explained presently. These lamps and theactuators for said switches are the only portions of the apparatusaccessible to persons desiring to unlatch the door for the purpose ofgaining entry to the restricted portion of the premises.

Save for the external station A and the monitoring station M, all of theapparatus shown in FIGURES l and 2 with the exception of solenoid S andswitches E and D conveniently can be located in a junction box in therestricted area, preferably on a wall close to the latched door.

Solenoid S is part of the latching mechanism and is effective, whenenergized, to release the latch so that the door can be opened. Thelatching mechanism itself is conventional, being of the type having aspring-biased bolt or latching member which normally holds the doorclosed, but which can be released by actuation of solenoid S.

Switch E is a push-button type exit switch located close to the door ina position where it is readily accessible to those who are alreadyinside the restricted room, but not, of course, to persons outside theroom. Assume that a person inside the room desires to leave. Simply bypressing the exit switch E, he will complete an obvious circuit forenergizing the latch solenoid S, thus releasing the latch so that he canopen the door and depart. When the door swings closed behind him, itwill automatically relatch under the influence of the spring-biasedlatch mechanism, and he then can reenter the room only by registeringthe proper code at the external station A, as described hereinafter.

For supervisory and other purposes which will be made clear presently,switch D is actuated by the door itself. Opening of the door serves toopen the switch and cause it to remain open until the door is reclosed;stated differently, switch D is closed at all times except during suchintervals as the door is open.

The external station A has a start switch 11a and 11b, and tencode-selection switches numbered consecutively 1, 2, 3 9 and 0.Conveniently, all of these can be push button switches of thespring-biased self-restoring type, whereby each switch always remainsopen, as shown, except during the interval that pressure is manuallyapplied to the associated push button. Code selection is elfected bypressing certain ones of the numbered push buttons in a predeterminedsequence.

The code which will serve to unlatch the door may have as many digits asdesired, but for purposes of explanation it will be assumed that afour-digit code is used; according to the invention this requires threeconventional bistable flip-flop circuits FF-l, FF2 and FF-3 togetherwith an associated three-stage series AND gate G.

Each of the flip-flops is reversible between two conditions undercontrol of positive pulses applied to one or the other of two inputs.More particularly, assuming negative potential is connected to conductor22, a positive pulse applied to in uts 27, 28 and 29 establishes astable condition in which transistors Q Q and Q are ofP' ornonconducting, While transistors Q Q and Q, are on." In this condition(which conveniently can be referred to as the reset condition of theflip-flops) the base of each gate transistor (Q Q and Q is biasednegatively so these transistors are disabled.

A positive pulse applied to input 31 of fiip-fiop FF-1 will reverse itscondition, changing it to what may conveniently be referred to asregister" condition. In this condition transistor Q; is on and Q is offand with Q, conducting, the base of transistor Q is biased positively sothis transistor is enabled, which is to say it is conditioned to becomeconductive if and when positive potential is applied to its collector.

Similarly, a positive pulse applied to input 32 of flipflip FF-2 willplace this flip-flop in register condition (wherein Q is on and Q; isofi) and, as a consequence, will enable gate transistor Q In likefashion, a positive pulse applied to input 33 of flip-flop FF-3 willplace it in register" condition, which enables gate transistor Q As willbe seen presently, flip-flop FF-l will go into register condition onlyif the first digit of the required four-digit code is correctly selectedat the external station A by the person seeking entry. Likewise,flip-flop FF-2 will go into register condition only if the second digitis correctly selected; and flip-flop FF-3 will do so only if the thirddigit is correctly selected and registered. Only when theseprerequisites have been satisfied will correct selection of the fourthdigit of the code cause the door to be unlatched.

The particular digits in the required code are determined by a multi-pinplug P which is removably received in a matching multi-terminal jack I.It is the function of the plug, among other things, to couple theregister inputs 31, 32 and 33 of the three flip-flops to the codeselection switches at station A which correspond respectively to thefirst three digits of the required code, and to couple all noncodeswitches to the flip-flop reset lead 35. In the case underconsideration, the pins of plug P are iumpered so as to establish thedigital sequence 2-5-7-8 as the code which will unlatch the door, but itwill be understood that this is only exemplary and that other plugs(interchangeable with plug P) can be wired to set up any other fourdigit code that may be desired.

With plug P in place in jack I, let it be assumed that a person familiarwith the required code, 2-5-7-8, Seeks to enter the restricted orprivate" area via the latched door which is associated with externalstation A. To gain entry, he must first momentarily depress the startpush button, closing switch contacts 11a and 11b. When this is done, acircuit is completed which extends from positive battery via conductor12, switch contact 11a, conductor 14, winding of relay 15, conductor 16,switch D, conductor 18 and switch 19 to negative battery. Relay 15 thusoperates, closing its contacts.

The negative potential on conductor 16 (from contact 19) therefore isnow connected by relay contact 15d to conductors 21 and 22, thussupplying negative battery to the three flip-flop circuits. The negativepotential on conductor 16 also is connected, by realy contact 15c, tothe heater 24 of a thermal time delay relay TD, thereby to initiatemeasurement of a predetermined time interval for completing the code andopening the door. Relay contact 15b completes an obvious locking circuitfor relay 15 to insure that it will remain operated when the startswitch 11a, 11b is allowed to open; and contact 15a completes an obviouscircuit for lighting the start lamp 25 at the external station A.

In addition to the foregoing, the momentary closing of the start switchcompletes a circuit which extends from positive battery via conductor12, switch contact 11b and conductor 26 to reset conductor 35, so that apositive pulse is ap lied to the inputs 27, 28 and 29 of the threebistable flipfiop circuits. Consequently, as explained hereinbefore,transistors Q Q and Q are turned ofF while transistors Q Q and Q areturned on"; and with transistors Q Q and Q in off condition, the stringof transistors Q Q and Q, in the AND gate (3 are disabled.

Lighting of lamp 25 at the external station A upon momentary operationof the start switch, as described above, indicates to the person seekingentry that the apparatus has been conditioned to receive the code andthat the correct code (which in the exemplary case under considerationis 25-7-8) must be registered within a predetermined brief itnerval oftime. For a four digit code, the time delay relay TD may, for example,be set to allow an interval of approximately ten seconds for coderegistration.

The correct code is registered, of course, by momentarily operatingswitches 2, 5, 7 and 8 in that order. Operation of switch 2 momentarilyconnects the positive potential on line 12 to conductor 2a and thencethrough pin 2p, jumper conductor 31a, pin 31p, to the input 31 of theflip-flop circuit FF-l. This positive pulse converts the flip-flop toregister condition, turning transistor Q off and transistor Q on,whereby gate transistor Q, is enabled. As Q turns on, the rise in itscollector current also delivers a positive spike through couplingcapacitor 38 to the flip-flop circuit FF-Z, but this is without elfectas transistor Q already is in off condition.

When the person seeking entry now operates switch 5 at the externalstation A, this momentarily connects the positive potential on line 12to conductor 5a and thence through pin 5p, jumper conductor 32a and pin32p to input 32 of the flip-flop circuit FF-Z. The effect of this issimilar to that just described in connection with flip flop FF-l, whichis to say that transistor Q turns off while transistor Q turns on,delivering a positive spike to fiipflop circuit FF-3 through couplingcapacitor 39 and biasing the base of gate transistor Q so that thistransistor is enabled.

In similar fashion, when switch 7 at the external station A is operated,this momentarily connects positive potential from line 12 to conductor7a and thence through pin 7p, jumper conductor 33a and pin 33p to theinput 33 of the flip-flop circuit FF-3. Accordingly, this flip-flop alsois changed to register condition and its associated gate transisaor Q,;is enabled. The AND gate G now is fully conditioned as a result ofcorrect registration of the first three digits of the code.

When the person seeking entry now closes switch 8 to register the lastdigit of the code, this completes a circuit which extends from positivebattery through switch 8, conductor 8a, pin 8p, jumper conductor 34a,pin 34p, conductor 34, winding of relay 40, conductor 42, the seriesstring of transistors comprising AND gate G, conductors 44, 22 and 21,relay contact 15d, conductor 16, switch i D, conductor 18 and switch 19to negative battery. En-

ergized over this circuit, relay 40 operates, closing its contacts.

Relay contact 40a now completes an obvious locking circuit for therelay, to insure that it will remain operated when switch 8 is allowedto open. Relay contact 40c completes a circuit for solenoid S, thusreleasing the door latch so that the door may be opened; and contact 40bcompletes an obvious circuit for the enter" lamp 45 at the externalstation A to indicate that the door may now be opened.

As previously explained, opening of the door causes switch D to open.This removes negative potential from conductors 16, 21 and 22 thuscausing relays and 40 to release. Accordingly, lamps and 45 at theexternal station A are extinguished and all potential is removed fromtransistors Q; to Q inclusive, and their associated circuit elements.Also, the circuit of heater 24 of the time delay relay TD is opened andthis relay restores to normal, having been without effect since, in thecase under consideration, it is assumed that the door was opened withinthe prescribed interval of time after closing of the start switch 11a,11b. Upon reclosing of the door, the system is restored to originalcondition, and the door then can be opened again only by repetition ofthe operation described hereinbefore.

It will be convenient at this point to refer to the door monitoringstation M. This has a pair of transistors Q and Q with a door conditionindicator lamp 46 in the collector circuit of one and a lock out lamp 48in the collector circuit of the other. The emitters of both transistorsare connected to negative battery through an audible signal device oralarm 50.

Under normal conditions, the negative bias applied to the base of bothtransistors maintains them in otP condition so that lamps 46 and 48 arenot lighted. Opening of the door switch D, however, removes the negativebias from transistor Q causing it to turn on. Accordingly, lamp 46 islighted and the audible signal 50 is energized, a condition whichcontinues until closing of the switch D restores the negative bias tothe base of transistor Q turning it off. The security officer or otherattendant at station M thus is appraised exactly when the door isopened, exactly when it is closed and just how long it remains open ineach instance. In the event the door stays open longer than is normal,appropriate investigation of the cause can be made at once.

Let it be assumed for a moment that the person seeking entry, afterregistering the correct code (whereby the door is unlatched byenergization of solenoid S and the enter lamp 45 is lighted) is calledaside and, without opening the door, walks away leaving the doorunlatched or alternatively, that he leaves after registering only partof the code, in which case the door will not be unlatched but one ormore of the flip-flops will be left in register condition.

The interval allowed by time delay relay TD for opening the door willrun out almost immediately after complete registration of the code and,in the event of incomplete registration of the code, within only a fewseconds; when the allowed interval runs out, contact 24a closes,connecting positive battery via conductor 20, resistor 23, conductors 26and to the inputs 27, 28 and 29 of the three flip-flop circuits. Thesetherefore are reset to their original condition, that is to saytransistors Q Q and Q are turned off while transistors Q Q and Q; areturned on. As a result, transistors Q Q and Q of the AND gate G likewiseare turned off, so that relay 40, it previously operated, releases andopens its contacts. The opening of relay contact c deenergizes solenoidS, restoring the door to latched condition; and the enter lamp isextinguished due to opening of contact 40b.

The operation of time delay relay TD also connects positive battery viacontact 24a and conductor 20 to the base of transistor Q turning thistransistor on. Accordingly, the lock out lamp 48 at the monitor stationM is lighted and the audible signal 50 is actuated, calling to theattention of the security oflicer that the door control system has beenlocked out or inactivated for reasons that should be investigated. Afterappropriate investigation, he can restore the system to operation bymomentarily opening the manual switch 19 at the monitor station M. Byremoving negative potential from conductor 16, this causes relays 15 andTD to restore to normal, causes lamp 25 to be extinguished, and causesall potential to be removed from the flip'fiop circuits FF-l, FF2 andFF-3. When the manual switch 19 is reclosed by the security officer, thesystem is once more conditioned for operation.

Remembering that the correct code is 24-7-8, let it now be assumed thatafter momentarily pressing the start switch 11a, 11b, the person seekingentry registers the correct digits but in the wrong order, for example,7852. Upon closing switch 7, this will apply positive potential to input33 of the flip-flop FF3, so that transistor Q, is turned off and Q isturned on, enabling transistor Q as previously described. When switch 8next is closed this will apply positive potential via conductor 34 torelay 40, but the relay cannot operate inasmuch as transistors Q and Qhave not been properly conditioned; accordingly the second digit of theattempted code is lost."

When the switch 5 next is closed, this applies positive potential to theinput 32 of flip-flop circuit FF-Z, turning transistor Q off and Q, on.The positive spike which now is delivered to flip-flop FF-3 throughcoupling capacitor 39 as transistor Q turns on resets FF-3 to itsoriginal condition, which is to say that transistor Q is turned off andQ on, so that gate transistor Q also is disabled. Likewise, when theperson seeking entry closes switch 2, this applies positive potential tothe input 31 of flip-flop circuit FF-l which reverses the conditionthereof, turning transistor Q off and Q, on, with the result that thepositive spike delivered to flip-flop FF-Z through capacitor 38 erases"the digit previously registered thereon.

If the person seeking entry closes any of the switches 1, 3, 4, 6, 9 or0 (that is to say, any switch corresponding to a digit which is not inthe code) positive potential will be applied via the correspondinglynumbered pin of plug P, conductor 35a, pin 35p and conductor 35 to theinputs 27, 28 and 29 of the three flip-flop circuits. This has theeffect of resetting to their original starting condition any of theflip-flop circuits which previously have been re versed as a result ofprior operation of one or more of the switches 2, 5 or 7. In otherwords, if any one or more correct digits of the code are registered, theregistration is automatically erased in the event it is followed byclosing of a code selection switch corresponding to a noncode digit.Likewise, as we have seen, even if the correct digits are registered,but out of order, the later registration will erase the effect of theearlier one.

Since the erasures or cancellaitons resulting from errors in codeselection occur without knowledge of the person seeking entry, there isvirtually no chance of arriving at the correct code within the allottedtime interval through random choice of the code-selection switches.Attempts to do so are cut short by the time delay relay TD which, aspreviously explained, signals the attendant at the door monitoringstation M by lighting lamp 48 and locks out the system against furtherregistration of digits until such time, following investigation, as itis deemed appropriate to clear the condition by momentarily openingswitch 19.

It is contemplated that other plugs, interchangeable with plug P, willbe prewired to establish other codes for unlatching the door. In thisconnection, it will be noted that pins 1p, 2p, 3p 10p of the plugcorrespond respectively to the digits 1, 2, 3 0. To set up a differentcode, say 30-7-4, pin 3p would be jumpered to pin 31p; pin 10p jumperedto pin 32p; pin 7p jumpered to pin 33p; pin 4p jumpered to pin 34p; andpins 1p, 2p, 5p, 6p, 8p

and 9p, jumpered to pin 35p. Stated diiferently, pins 31p 32p, 33p and34p are always connected, respectively, to the pins corresponding to thefour digits of the desired code, while the pins corresponding to allother (i.e., noncode) digits are connected to pin 35p.

Also, it should be noted that the same digit may be used more than oncein the code. For a code such as 15-52, for example, pins 32p and 33pwould both be jumpered to pin 5p; and for the code 5552 pins 31p, 32pand 33p all would be jumpered to pin 5p of the plug.

By having a number of interchangeable plugs prewired for different codesit is only necessary for a security officer to remove one plug andsubstitute another in jack J in order to change the code which will beeffective to unlatch the door. Thus the code can be changed very easilywhen desired, and as frequently as the circumstances warrant, which insome installations may be daily or oftener.

From the foregoing it will be seen that the invention is one welladapted to attain all of the ends and objects hereinbefore set forth,together with other advantages which are obvious and which are inherentto the apparatus.

It will be understood that certain features and subcombinations are ofutility and may be employed without reference to other features andsubcombinations. This is contemplated by and is within the scope of theappended claims.

Inasmuch as various possible embodiments of the invention may be madewithout departing from the scope thereof, it is to be understood thatall matter herein set forth or shown in the accompanying drawings is tobe interpreted as illustrative and not in a limiting sense.

Having thus described my invention, I claim:

1. In a system for releasing a door latch, a group of digit selectionswitches, a plurality of bistable flip-flop circuits each having aregister input for setting the circuit in the first stable state and areset input for setting the circuit in the second stable state, a gatecircuit controlled by said flip-fiop circuits and enabled only when allof the flip-flop circuits are in said first stable state, a relay, meansconnecting said relay and gate circuit in series to a particular one ofsaid digit switches, means connecting said register inputs to selectedother ones of said circuits, means connecting said reset inputs to stillother ones of said switches, and latch releasing means operated respon.sive to energization of said relay.

2. A system as in claim 1, wherein said connecting means comprises amulti-terminal jack and code-assigning plug removably received in saidjack.

3. A system as in claim 1, having a door operated switch, and meanscontrolled by said switch for deenergizing said relay responsive toopening of said door.

4. A system as in claim 1, having a door operated switch, and meanscontrolled by said switch for disabling said gate circuit responsive toopening of the door.

5. In a system for releasing a door latch, a group of digit selectionswitches, a plurality of bistable flip-flop circuits each having a firststable state and a second stable state, means for initially conditioningsaid circuits to said first stable state for operation under control ofsaid switches, other means for setting said circuits in said firststable state responsive to operation of certain of said switches, saidcircuits corresponding respectively to certain other ones of saidswitches, means for setting each circuit in said second stable stateresponsive to operation of its corresponding switch, means for releasingsaid latch when all of said circuits are in said second stable state,and means controlled by said circuits for disabling said latch releasingmeans whenever any one or more of said circuits is in said first stablestate.

6. A system as in claim 5, wherein at least one of Said flip-flopcircuits has means for setting that circuit in said first stable stateresponsive to a change of another one of said circuits from said firststate to said second state.

7. A system as in claim 5, having a signal, and door controlled meansfor operating said signal and disabling said flip-flop circuits wheneverthe door is open.

8. A system as in claim 5, wherein said flip-flop circuits are normallydisabled, and time delay means effective a predetermined interval oftime after the conditioning of said circuits for setting and holding allof said circuits in said first stable state.

9. A system as in claim 8, having door controlled means for disablingsaid time controlled means responsive to opening of the door.

10. In a system for releasing a door latch, a group of digit selectionswitches, a plurality of bistable flip-flop circuits each having a firststable state and a second stable state, said circuits correspondingrespectively to certain of said switches, means operable at will tocondition said circuits for operation and simultaneously to set them insaid first state, means for thereafter shifting each circuit to saidsecond state responsive to operation of its corresponding switch, meansfor releasing said latch when all of said circuits are in said secondstable state, and means controlled by said circuits for disabling saidlatch releasing means whenever any one or more of said circuits is insaid first stable state.

11. In a system for releasing a door latch, a group of digit selectionswitches, a plurality of bistable flip-flop circuits each having a firststable state and a second stable state, said circuits correspondingrespectively to certain of said switches, means operable at will tocondition said circuits for operation and simultaneously to set them insaid first state, means for thereafter shifting each circuit to saidsecond state responsive to operation of its corresponding switch, arelay, means under control of said circuits for energizing said relayonly when all of said circuits are in said second stable state, andlatch releasing means operated responsive to energization of said relay.

12. In a system for releasing a door latch, a group of digit selectionswitches, a plurality of bistable flip-flo circuits each having a firststable state and a second stable state, means operable at will tocondition said circuits for operation and simultaneously to set them insaid first state, means for setting said circuits in said second stateresponsive to operation of certain other ones of said switches inpredetermined sequence, a relay, means under control of said circuitsfor energizing said relay only when all of said circuits are in saidsecond state, and latch releasing means operated responsive toenergization of said relay.

13. A system as in claim 12, wherein said flip-flop circuits areconcatenated, each having means for setting that circuit in said firststate responsive to a change of the next-preceding circuit from saidfirst to said second state.

14. In a system for releasing a door latch, a group of digit selectionswitches, a plurality of bistable flip-flop circuits each having a firststable state and a second stable state, means operable at will tocondition said circuits for operation and simultaneously to set them insaid first state, means for setting said circuits in said first stateresponsive to operation of certain of said switches, means for settingsaid circuits in said second state responsive to operation of certainother ones of said switches in predetermined sequence, a normallydisabled relay associated with still another one of said switches, meanscontrolled by said flip-flop circuits for enabling said relay only whenall of said circuits are in said second state, means for energizing saidrelay, after thus enabled, responsive to operation of the switch withwhich said relay is associated, and latch releasing means operatedresponsive to energization of said relay.

15. A system as in claim 14, wherein said flip-flop circuits areconcatenated, each having means for setting that circuit in said firststate responsive to a change of the next-preceding circuit from saidfirst to said second state.

16. A system for releasing a door latch, said system comprising, a groupof digit selection switches, a plurality of concatenated bistableflip-flop circuits each having a first stable state and a second stablestate, means for initially conditioning each of said circuits to saidfirst stable state for operation under control of said switches, meansfor setting said circuits in said first state responsive to operation ofcertain of said switches, means for setting said circuits in said secondstate responsive to operation of certain other ones of said switches inpredetermined sequence, a relay, gate means controlled by said flip-flopcircuits for enabling said relay only when all of said circuits are insaid second state, means for resetting said circuits responsive to saidoperation of said switches out of said sequence to preclude operation ofsaid enabling gate means and latch releasing means operated responsiveto energization of said relay.

References Cited JOHN F. COUCH, Primary Examiner.

10 J. D. TRAMMELL, Assistant Examiner.

US. Cl. X.R.

